Processing of workpieces using a laser has been practiced from before. For example, metal processing, in particular, welding of steel plates by a laser is employed widely in many industries, such as the automobile industry. FIG. 1 shows a manner in which two steel plates 71 and 72 are laser welded. As shown in FIG. 1(a), in CO2 laser welding or YAG laser welding, ends of steel plates 71 and 72 are butted against each other and a laser beam 74, having a substantially circular transverse section, is illuminated onto a butted region 73. FIG. 1(b) shows intensity profiles of the laser beam 74. If a coordinate system, having an A-axis, extending along a longitudinal direction of the butted region 73, and a B-axis, perpendicular to the A-axis, is defined, the laser beam 74 has a Gaussian intensity profile in both the A-axis direction and the B-axis direction.
As the light source for laser welding, a semiconductor laser element can be used as disclosed in Japanese Published Unexamined Patent Application No. Hei 8-309574. Normally with a laser beam that is emitted from a semiconductor laser element, because a spread angle in a fast axis direction differs from that in a slow axis direction, the laser beam has a transverse section of elongate shape. With the invention described in the abovementioned Publication, a synthesized beam with a transverse section that is close to being circular is formed by overlapping laser beams from a plurality of semiconductor laser elements with the longitudinal directions of the transverse sections of the respective beams being made to differ from each other.
Laser welding can also be performed using a laser beam of high output (no less than several hundred W) emitted from a semiconductor laser array. Because a semiconductor laser array has a plurality of light emitting portions that are aligned in a slow axis direction, a laser beam 75 from the semiconductor laser array has a rectangular transverse section as shown in FIG. 1(c). By matching a major axis direction of the laser beam 75 with the longitudinal direction of the butted region 73 and scanning the laser beam 75 in the major axis direction, welding can be performed while performing preheating and slow cooling. Because thermal strain can thereby be suppressed, a high processing quality is obtained.
FIG. 1(d) shows intensity profiles of the laser beam 75 in the major axis direction and the minor axis direction. The profile in the major axis direction of the laser beam 75 is formed by the overlapping of Gaussian profiles of laser beams emitted from the respective light emitting portions. The laser beam 75 thus has a top hat shaped profile, with a flat peak portion, in the major axis direction. Meanwhile, the profile in the minor axis direction has the same Gaussian shape as a laser beam emitted from a single light emitting portion. A laser beam having such profiles is disclosed in Japanese Published Unexamined Patent Application No. 2002-335035.
FIG. 2 is an enlarged view of the butted region 73 shown in FIG. 1. It is difficult to form the ends of the steel plates 71 and 72 into perfect straight lines and the ends are curved or deformed in many cases. Thus when the ends of two steel plates are butted against each other, a gap may form between the two. A width W of this gap is extremely non-uniform (for example, approximately 0 to 200 μm).
Because each of the abovementioned laser beams 74 and 75 has Gaussian profile along the direction that extends across the butted region 73 (B-axis direction), allowances in regard to the gap between the steel plates and the deviation of the beam illumination position are small. For example, a portion of the laser beam that is highest in intensity may become illuminated onto the gap, thereby lowering the welding efficiency and causing a processing fault. Also, because normally the illumination position of a laser beam has an error of approximately ±50 to 100 μm in the B-axis direction, even a slight deviation of the illumination position may cause the high intensity portion of the laser beam not to be illuminated onto the butted region 73 and thereby cause a processing fault.
Thus an object of the present invention is to provide a laser apparatus that generates a laser beam, with which a processing fault is unlikely to occur during laser processing.